Airgel consists of. Homemade silicate airgel. Preparation of silicone airgel by subcritical drying

Since it was said about aerogels, here is some development of the topic. This is a translation of a blog post by a person who made silicate airgel in his workshop using a recipe from the website.

TMOS is quite difficult to obtain because chemical suppliers are very reluctant to sell anything to individuals.

Airgel manufacturing process:

1. Mix TMOS, methanol and ammonium hydroxide. Pour the mixture into molds and let the gel harden.
2. Immerse the gel in methanol and wait a day for the remaining water in the gel to dissolve in the methanol.
3. Discard the used methanol and replace it with clean methanol. Wait another day and repeat the process. This will need to be done several times over three days.
4. Move the gel to supercritical drying chamber and fill it with methanol.
5. Add liquid carbon dioxide, open the chamber drain valve and drain the methanol. Make sure that the gel pieces are in liquid CO2 at all times.
6. Wait a day for the methanol to dissolve in the liquid CO2.
7. Open the drain valve and pour out some more methanol that has dissolved in CO2.
8. Repeat the procedure for draining the methanol, but make sure that the gel remains immersed in liquid CO2 at all times. Repeat draining/replacing CO2 a couple of times over 2-3 days.
9. Raise the chamber temperature until the CO2 becomes supercritical. Slowly open the vent while continuing to heat the chamber to allow the CO2 to go from supercritical to gaseous state. Slowly release all the CO2 from the chamber, then remove the finished airgel from it.

From the comments:

jstults wrote...
Why are they so afraid to sell TMOS to individuals?

Ben Krasnow wrote...
jstults, most chemical suppliers will not sell anything to individual customers - regardless of whether the substance is dangerous or not. Accident or improper use chemical preparation can lead to a lawsuit and have a bad impact on the reputation of the supplier, so the management of many companies decided that it was more profitable for them to forget about the money individuals, but avoid the associated risks. Bad news for those doing science at home, but that's unlikely to change. The best we can hope for is that hackerspaces establish themselves well enough to be able to order chemicals for their members. Now, I suspect that many hackerspaces will encounter difficulties when trying to order something from companies like Sigma Aldrich.

Will Walker wrote...
Hello Ben -

Great job with aerogels and documentation. Here are a few tricks you could try to reduce cracks:

1. Fill the dryer to the top with MeOH before you begin to pressurize. You want to make sure that when you increase the pressure, the gel is completely covered by the liquid.

2. First release the compressed methanol in portions (20-30% at a time) intermittently before replacing with pure CO2. Mixing different solvents creates a pressure difference within the gel as pure MeOH tries to evaporate. This is actually the main “mysterious” source of cracks.

3. Contrary to what intuition would suggest, it appears that thermal expansion is NOT main reason appearance of cracks in aerogels. What is really important in this case is the rate at which the pressure rises and falls. It is also important to initially slow down the replacement of MeOH with CO2, as described above.

I followed the instructions from the TMOS (tetramethyl orthosilicate) recipe found at http://www.aerogel.org and successfully made several pieces of aerogel in my home workshop.
Two things presented the greatest challenge: 1. Getting TMOS or TEOS (the key chemical ingredient) and 2. Building a supercritical drying chamber. Chamber parts can be purchased from http://www.mcmaster.com or any other industrial pipe fitting supplier. You will also need a supply of liquid carbon dioxide. I used a 20 lb (9.1 kg) cylinder purchased from a local welding supply store. Most the cost was precisely the cylinder itself, Consumables It only cost $20 or $30. You may be able to find a supplier from whom you can rent a cylinder.

TMOS is quite difficult to obtain because chemical suppliers are very reluctant to sell anything to individuals.

Airgel manufacturing process:

1. Mix TMOS, methanol and ammonium hydroxide. Pour the mixture into molds and let the gel harden.
2. Immerse the gel in methanol and wait a day for the remaining water in the gel to dissolve in the methanol.
3. Discard the used methanol and replace it with clean methanol. Wait another day and repeat the process. This will need to be done several times over three days.
4. Transfer the gel to the supercritical drying chamber and fill it with methanol.
5. Add liquid carbon dioxide, open the chamber drain valve and drain the methanol. Make sure that the gel pieces are in liquid CO2 at all times.
6. Wait a day for the methanol to dissolve in the liquid CO2.
7. Open the drain valve and pour out some more methanol that has dissolved in CO2.
8. Repeat the procedure for draining the methanol, but make sure that the gel remains immersed in liquid CO2 at all times. Repeat draining/replacing CO2 a couple of times over 2-3 days.
9. Raise the chamber temperature until the CO2 becomes supercritical. Slowly open the vent while continuing to heat the chamber to allow the CO2 to go from supercritical to a gaseous state. Slowly release all the CO2 from the chamber, then remove the finished airgel from it.

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Airgel - very unusual creation human hands, a material awarded 15 positions in the Guinness Book of Records for its unique qualities. The name "aerogel" comes from two Latin words aer - air and gelatus - frozen. Therefore, airgel is often called “frozen smoke”.

However, according to appearance the airgel really resembles frozen smoke. Airgel is an unusual gel in which there is no liquid phase, completely replaced by a gaseous one, as a result of which the substance has a record low density, only one and a half times the density of air, and a number of other unique qualities: hardness, transparency, heat resistance, etc. Airgel is also surprising because it consists of 99.8%... air!
The history of the appearance of airgel is still not fully understood. It is only known that the American scientist Samuel Kistler was the first to receive it in the late twenties or thirtieth year of the last century at the College of the Pacific in Stockton (California). Obtained, as sometimes happens, in scientific research, almost by accident, as a by-product of the crystallization of amino acids in supercritical supersaturated liquids. The scientist achieved the production of an airgel by replacing the liquid in a regular gel with methanol. After this, the gel was heated under high pressure to 240 degrees (critical temperature for methanol). At this point, methanol gas left the gel, but the dehydrated foam did not decrease in volume. As a result, a lightweight, finely porous material was formed, later called airgel. The official date of appearance of the new material is considered to be 1931, the time of publication of an article about it in the journal Nature. The origin of the term “aerogel” is also unknown. It remains a mystery whether Kistler himself introduced it into our speech, or took advantage of the hint of his colleagues. The first airgel was obtained by a scientist from quartz. Subsequently, they learned to make this material from metal oxides, organic matter, and many other starting ingredients.
In structure, aerogels are a tree-like network of particles 2-5 nanometers in size united into homogeneous groups (clusters) and air-filled pores up to 100 nanometers in size. Externally, airgel most closely resembles transparent or translucent frozen soap foam. When viewed with the naked eye, airgel appears to be a solid, homogeneous substance, which distinguishes it from such porous media as various foams. The airgel also feels like frozen foam to the touch. This is enough durable material- the airgel can withstand a load of 2000 times its own weight. For example, a small block of airgel weighing 2.38 g can easily withstand a brick weighing 2.5 kg!

Quartz aerogels are a very good heat insulator. The process of producing aerogels is complex and labor-intensive. First, using chemical reactions, the gel polymerizes. This operation takes several days and the output is a jelly-like product. Then the water is removed from the jelly with alcohol. Its complete removal is the key to the success of the entire process. The next step is “supercritical” drying. It is produced in an autoclave at high pressure and temperature, the process involves liquefied carbon dioxide. The applied use of quartz airgel as an insulation material began in the forties of the twentieth century. Famous company Monsanto produced this product under a licensing agreement with Kistler. However, due to their high cost, airgel heat insulators were not widely used, and production was curtailed in the seventies. Only at the very end of the last century did aerogels again begin to be widely used by humanity, primarily in the space industry. It was aerogel that became the most important element Array catcher, which was used by the Stardust space probe to capture millions of tiny particles from the tail of comet Wild 2 and bring the lander with these samples to earth. By the way, among the variety of particles captured by the probe, traces of glycine were found, the most important amino acid for protein formation. For scientists who share the theory of the extraterrestrial origin of life, this find became indirect proof that they were right.
The airgel is planned to be used as a unique thermal insulator in American-made space suits created for NASA's Mars project. NASA also announced the use of airgel as a heat shield for new shuttle models. Aerogels are also promising in microelectronics. Mainly due to the fact that they have the lowest dielectric constants. The use of aerogels as insulating layers in multilayer printed circuit boards will significantly increase the performance of electronics. In 2007, American chemists presented the aerogels they had created, which can serve as a filter for purifying water from harmful impurities, such as mercury, lead and other toxic heavy metals. So far, the production of these materials is quite limited due to high price, because The filters contain platinum, but when a replacement is found for it in the form of a cheaper analogue, the new type of purifiers will be able to rid the planet’s water bodies of heavy metals. In addition, new aerogels exhibit semiconductor properties and, therefore, can be used in photovoltaic cells and other optoelectronic devices.

Quartz airgel, as already mentioned, is a unique heat insulator. It can withstand temperatures up to 500 degrees Celsius, and a 2.5cm thick layer is enough to protect the human hand from direct impact blowtorch. There are varieties of aerogels with a melting point of up to 1200 C. The properties of arogels largely depend on source material, from which they are made. There are aerogels made from alumina (with the addition of aluminum oxide), silicon dioxide, as well as tin and chromium oxide. More recently, carbon-based aerogels have been produced. There are aerogels used as catalysts. NASA is currently testing aluminum oxide aerogels containing rare elements - gadolinium and terbium. These aerogels are used as high-speed collision detectors. Some transparent varieties of aerogel are being considered by scientists as a replacement window glass. After all, the refractive index of aerogels is much lower than that of glass (1.05 versus 1.5). Science has already managed to overcome the initial fragility of this promising material; elastic and flexible aerogels are now available. On the agenda is the issue of reducing production costs to limits that make use on a large scale profitable. Aerogels are often called the material of the 21st century. Whether this is so, we will soon see.

Airgel is extremely lightweight material with very low density and low thermal conductivity. It is translucent, but at the same time quite hard to the touch. Typically, aerogels are made using a supercritical drying process, so to make your own airgel, you will have to build a drying apparatus for such drying. There are ways to prepare aerogels without using such a device, but they are less reliable and the material turns out to be more dense. In this article you will find information about both methods of preparing airgel.

Steps

Part 1

Take a look at the supercritical dryer diagram above. Below you will find instructions for self-assembly such a device.

• The diagram is taken from the following web page: http://www.aerogel.org/wp-content/uploads/2009/02/manuclave-design2.jpg

1. Tightly connect two 316 or 304 stainless steel couplings and valves to a non-weldable stainless steel T-fit. The diameter of the fitting should be 3/4 inch (1.9 cm).

   • Tubular plugs (gates) must be connected to the two arms of the T-shaped connection.
   • If desired, instead of one of the plugs, you can install a viewing window.
   • Attach a ball valve to the bottom of the T-junction using a 6.35 mm (1/4 in.) bushing.
   • Screw the parts to the top outlet in the following order: 1.2 cm (1/2 inch) grommet, 1.2 cm (1/2 inch) nipple, and 1.2 cm (1/2 inch) cross-shaped pipe.

2. Complete the assembly of the upper part of the device. The remaining valves and sensors will be connected to this second pipe.

   • Connect a bimetallic thermometer to the upper outlet of the pipe.
   • Attach a 6.35 mm by 1.2 cm (1/4 inch by 1/2 inch) nipple to the left arm of the pipe. Attach a ball valve to it.
   • Connect another 6.35 mm by 1.2 cm (1/4 inch by 1/2 inch) nipple to the right arm of the pipe. Attach a 6.35 mm (1/4 inch) diameter pipe to it with a pressure gauge on the top and a safety spring valve on the bottom branch.
   • Attach a 6.35 mm (1/4 inch) nipple with a needle valve to the smaller outlet hole.

3. Use the right materials. Stainless steel is preferred because it is clean, durable, and has little corrosion resistance.

   • Sensors with brass threads and carbon steel valves can be installed.
   • Do not use brass or carbon steel couplings, or incorporate any parts made of material that cannot withstand 15 MPa (approx. 2,000 psi).

4. Connect a carbon dioxide cylinder to the supercritical dryer. The connection must be leak-free and ensure free flow of liquid carbon dioxide into the apparatus.

   • A diagram of such a connection is shown on the following web page: http://www.aerogel.org/wp-content/uploads/2009/02/gastank-1.jpg
   • Connect the cylinder in the following order: CGA320 male adapter, Teflon gasket, inlet nipple, inlet coupling, 6.35 mm (1/4 in.) adapter with internal thread Quick Disconnect, 6.35 mm (1/4 inch) Male Quick Disconnect Adapter, Hose high pressure 6.35 mm (1/4 inch) diameter with internal braided threads.
   • At the other end of the high pressure hose, connect a 6.35 mm (1/4 in.) female adapter and a 6.35 mm (1/4 in.) male adapter for quick disconnect.
   • Connect the last coupling to the inlet ball valve of the dryer. Now everything is connected.

   Part 2

   Preparation of silicone airgel

   1. Dilute concentrated caustic ammonia (ammonium hydroxide). Dilute 4.86 g, or 5.4 ml of concentrated caustic ammonia in 1000 ml of water in a glass or plastic bottle.

      • Label this bottle "caustic ammonia aqueous solution." The solution can be stored at room temperature in a sealed bottle for later use.

   2. Prepare a solution of tetramethoxysilane with methanol. Mix 10.2 g (10 ml) tetramethoxysilane with 7.82 g (10 ml) methanol in a glass beaker. Stir the solution.

      • Label this solution "alkoxide solution", or simply "solution A".
      • Please note that tetramethoxysilane is not easy to purchase because companies that sell the chemicals are generally reluctant to sell them to individuals. If you manage to find a company willing to sell this substance, make sure that it has a high degree of purity and does not contain dangerous impurities.

   3. Prepare a solution of caustic ammonia with methanol. Mix 5 g (5 ml) of the previously prepared aqueous solution of caustic ammonia with 7.92 g (10 ml) of methanol in another clean glass beaker. Stir the solution.

      • Label this solution "catalytic solution", or simply "solution B".

   4. Pour the catalytic solution into the previously prepared alkoxide solution. Carefully pour the catalytic solution (solution B) into the alkoxide solution (solution A) and stir them with a glass rod until completely dissolved.

      • The resulting solution is also called “sol”.

   5. Transfer the sol to the molds. Pre-lay silicone-based baking paper on the bottom of the molds. Wait until the sol takes the form of a gel.

      • You will have to wait from 15 minutes to 1 hour.
      • You can also pour the sol into small cylindrical pipettes. In this case, after the solution has turned into a gel, you can squeeze it out of the pipette.
      • Tetramethoxysilane in this method plays the role of a source of silicon oxide. Water causes tetramethoxysilane to polymerize, and methanol ensures that water and tetramethoxysilane mix together so that they form one phase and can react with each other. Caustic ammonia speeds up the reaction.

   6. Let the gel age. Once the gel has formed, place it in methanol and leave it there for at least 24 hours.

      Remove the water. Change the methanol at least four times during the week to fresh methanol, to ethanol with a purity of more than 99.5%, or to acetone.

      • As a result, the gel will be almost completely cleared of water.

   7. Dry the gel in a supercritical dryer. Place the gel in the chamber of the device and release carbon dioxide into it. The carbon dioxide will heat up and pass through its critical point, 31.1 degrees Celsius and 72.9 bar, warming up to about 45 degrees Celsius and reaching a pressure of about 100 bar.

      • During supercritical drying, the methanol will be completely removed from the gel.
      • Relieve the pressure in the apparatus chamber at a rate of about 7 bar per hour.
      • The result of the process is a silicon oxide airgel.

   Part 3

   Alternative process: subcritical drying

   1. Prepare the airgel as directed previously. Prepare silica airgel as described above by first preparing the chemical ingredients and then “soling” it to form a liquid gel.

      After soaking the gel in alcohol or acetone, clean it of water as described above. Change ethanol or acetone at least four times throughout the week.

      Prepare a solution of hexane and ethanol. Mix one part hexane and three parts ethanol to obtain a solution with a volume of at least five times the volume of the previous gel.

      • If desired, acetone can be used instead of ethanol in the same proportions.
      • For example, if you received 20 ml of airgel, you need to prepare a solution of 25 ml of hexane and 75 ml of ethanol or acetone.
      • Mark the container with the prepared solution as “solution 25-75”.

   2. Prepare two additional solutions of hexane and ethanol. In the second, use these two liquids in equal proportions. In the third, mix three parts hexane with one part ethanol.

      • As before, acetone can be used instead of ethanol.
      • Label the container with the second solution as “50-50 solution” and the container with the third as “75-25 solution”.

   3. Soak the gel sequentially in three solutions. Soak the gel in a solution of 25-75 for 12-48 hours.

      • Then place the gel in a 50-50 solution and keep it there for the same time, then put the gel in a 75-25 solution and keep it there for 12-48 hours.

   4. Soak the gel in hexane. Soak the gel for 24-72 hours in pure hexane, changing it three times.

   5. Prepare a solution of trimethylchlorosilane. Add trimethylchlorosilane to hexane so that it makes up 6% of the total mass of the solution.

      • Prepare a solution in a volume at least 15 times the volume of the previously prepared gel.
      • Label the container containing the solution as TMHS.

   6. Dip the airgel into the trimethylchlorosilane solution. To do this, first place the gel at the bottom of a tightly sealed chemical-resistant container with a wide neck, then pour enough TMCS solution into it so that its volume exceeds the volume of the gel being soaked by 5-10 times. Seal the container.

      • For a glass container, moisten the edges of the lid with a vacuum cleaner. silicone grease, otherwise the lid may stick to the neck.

   7. Warm up and then cool the container with the gel. Heat the vessel to 60 degrees Celsius and maintain it at this temperature for 12-24 hours, using an electric stove. Before replacing the TMCS solution with a fresh one, allow it to cool to room temperature.

      • Repeat the procedure twice more.
   8. Work in a well-ventilated area with good lighting.
   9. Wear safety glasses while working. Also wear long sleeves and closed shoes.
   10. Do not try to save on device parts by replacing them with cheaper ones.
   11. Devote enough time to work. Do not try to speed up the process, as this increases the likelihood of a dangerous error.
   12. Make sure all valves and connections are secure and tight and avoid contact with organic solvents. This will prevent carbon dioxide leaks.
   13. Change the gaskets to new ones after every 30-50 processes, and after the end of the process, close all valves tightly.
   14. Tetramethoxysilane is dangerous substance, which can harm your lungs and eyes, so when working with it, use safety glasses and a gauze bandage or a respirator.

   What you will need

   • Latex or rubber gloves
   • Protective glasses
   • Long sleeves
   • Closed shoes
   • Chemical apron

   Supercritical drying

   • 2 6.35 mm (1/4 in.) 316 stainless steel medium pressure ball valves
   • 2 hex nipples, 1.2 cm (1/2 in.) inlet, 6.35 mm (1/4 in.) outlet, 1.7 cm (1-11/16 in.) long, 316 stainless steel
   • Hex nipple 1.2 cm (1/2 inch)
   • 2 x 6.35 mm (1/4 inch) hex nipples
   • 1.2 cm (1/2 in.) 316 stainless steel hose
   • 6.35 mm (1/4 in.) 316 stainless steel hose
   • Medium pressure needle valve, female on both sides, 6.35 mm (1/4 in.) diameter, 316 stainless steel
   • Brass Spring Safety Valve with Control Pull Ring and Vent to Atmosphere, Male Thread, 6.35mm (1/4") Diameter
   • 1.2cm (1/2") 304 Stainless Steel Bimetal Thermometer with External Threads and Oil-Free Dial
   • Pressure gauge 0-20000 KPa, top connected, with 6.36 mm (1/4 inch) socket
   • Heater or hair dryer
   • Pipe insulation tape
   • Cross-shaped pipe
   • 9kg carbon dioxide cylinder with valve and adapter CGA320

   Silicone airgel

   • Tetramethoxysilane
   • Methanol
   • Demineralized water
   • 28-30 wt. % solution of caustic ammonia in water
   • Ethanol (possibly)
   • Acetone (possibly)

   Preparation of silicone airgel by subcritical drying

   • Prepared silicone gel
   • Pure ethanol or acetone
   • Hexane
   • Trimethylchlorosilane (TMCS)
   • Chemical-resistant wide-mouth jar or bottle
   • Electric stove
   • Chemical extract
   • Hexamethyldisilazane